Fossi put the telemarketing tactic in context.

"Fake AV is a much bigger problem than this," Fossi said. "This sounds like a pretty low rate of return on investment [for the criminals] because of the time commitment to do this. But maybe they're getting a higher success rate because [the victim] is actually talking to a real person. Some probably think, 'Why would they take the time to call if there wasn't a problem?'"

Traditional security software won't help users deflect phone calls from fake support technicians, Fossi acknowledged. "It's social engineering, so the only thing you can do is try to educate people," he said.

Microsoft urged Windows users to "be suspicious of unsolicited calls related to a security problem, even if they claim to represent a respected company," and gave other common sense advice, such as to never divulge credit card or bank account information to out-of-the-blue callers.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at or subscribe to . His e-mail address is .